Fourth Power just closed a $20 million Series A Plus to build what could be the energy industry's next breakthrough - thermal batteries that store electricity in molten tin at 2,400°C. The Cambridge startup claims its sci-fi approach will deliver grid-scale storage at $25 per kilowatt-hour, potentially one-tenth the cost of lithium-ion while lasting twice as long.
Fourth Power just turned energy storage into something out of a James Bond film. The Cambridge startup closed a $20 million Series A Plus round to build thermal batteries that store electricity using molten tin heated to a scorching 2,400°C - hot enough to melt copper twice over. Munich Re Ventures led the round, with Breakthrough Energy Ventures and DCVC participating, bringing the company's total funding to $39 million since its previous $19 million Series A in 2023.
The timing couldn't be better. Grid operators are scrambling for long-duration storage solutions as solar and wind power floods the system during peak hours but disappears when the sun sets. Current lithium-ion batteries typically discharge for 4 hours max - not enough to bridge the gap between afternoon solar peaks and evening demand spikes.
Here's where Fourth Power's approach gets interesting. Instead of cramming more lithium into boxes, they're essentially building thermal vaults. Electricity from the grid heats carbon blocks inside argon-filled chambers. When power's needed, the system pumps molten tin through graphite pipes - the only material that can handle those extreme temperatures without breaking the bank. Special thermophotovoltaic cells then convert the infrared radiation back into electricity.
"Our projections are for the first-of-a-kind — the first ones to market — that they'll be cost competitive," CEO and co-founder Arvin Ganesan told TechCrunch exclusively. "Our comps for these markets are very high."
Those "comps" include peaking natural gas plants, which utilities fire up when demand spikes and renewable sources fall short. Fourth Power's batteries can discharge for 8+ hours daily - twice as long as most grid-scale lithium systems. The insulation, made from petroleum coke (basically oil refining waste), keeps the system stable while losing just 1% of stored energy per day.
The real game-changer is the projected cost: $25 per kilowatt-hour at scale. That's roughly one-tenth the cost of lithium-ion batteries and could make renewable energy viable for 24/7 baseload power. "There's not a lot of moving pieces. There's not a lot of players in the supply chain, which means that you can get to cost targets in a fairly straightforward way," Ganesan explained.
Fourth Power is currently stress-testing smaller versions of the technology, running charge-discharge cycles to validate durability and power output. The fresh funding will build a 1-megawatt-hour demonstration unit - their first full-scale battery complete with the molten tin system.
The energy storage market is heating up fast. Companies like Form Energy are pursuing iron-air batteries, while others bet on compressed air or gravity systems. But Fourth Power's thermal approach offers unique advantages: no rare earth materials, no supply chain bottlenecks, and the ability to scale manufacturing without competing for lithium or cobalt.
Utilities are taking notice. The demonstration unit will serve as a proving ground for commercial customers eyeing 2028 deployments. If Fourth Power hits its cost targets, thermal batteries could finally make renewable energy a true 24/7 baseload option - no natural gas backup required.
The four-year-old startup has been quietly refining its technology for two years, but now they're ready to turn up the heat. With molten tin flowing at temperatures that would vaporize most metals, Fourth Power is betting that sometimes the most elegant solutions look like pure science fiction.
Fourth Power represents the kind of moonshot thinking the energy transition desperately needs. While competitors chase incremental improvements to lithium-ion technology, this Cambridge startup is rewriting the playbook entirely. If their molten tin thermal batteries deliver on cost projections, we could be looking at the technology that finally makes renewable energy a 24/7 reality. The demonstration unit will be the critical test - proving whether sci-fi energy storage can become grid-scale economics.
